1. Field of the Invention
The present invention relates to a speaker system and is particularly concerned with a speaker system which is small, directive with respect to sound emission, and which provides a full frequency range from a small speaker.
2. Description of the Prior Art
A speaker system is disclosed in Australian Pat. No. 143,597 in which a speaker is mounted in the upper end of a tube and directed toward a diffusion cone. The tube is mounted on legs so that the lower end thereof is located above the floor. With the exception of a plurality of struts, each having a central opening, the tube is hollow and lined with sound absorbing material. The tube is constructed of an arcuate plywood member with a generally flat, but inwardly curved rear panel so that the tube may stand close to a wall. The diffusor or reflector cone is mounted above the speaker to provide approximately 180.degree. sound distribution at the high frequencies.
The theory of operation of this speaker system is that, if the speaker with a certain fundamental low frequency resonance is located at the end of a column of length of approximately 1/4 of the wavelength (in air) at that frequency (approximately because wavelength varies with pressure and also effective column length is affected by "fringing" at an open end), then at that frequency the acoustic impedance of the column is infinite. In theory, the speaker resonance is damped and is replaced by two less violent resonances, one above and one below the speaker resonance. In demonstrating the above, the Australian patent compares the same with an analogous electrical series resonance circuit with reference to FIG. 3 of that patent. At low frequencies, the equivalent electrical circuit of the mechanical portion of a speaker comprises a capacitor analogous to the compliance of the cone suspension, an inductance analogous to the cone mass, and a resistance analogous to the mechanical resistance to motion, and a further resistance and inductance which are in series with the foregoing elements and parallel to one another and analogous to the air load. Furthermore, an analogous electrical circuit of the column comprises a resistor, an inductance and a capacitance connected in parallel which are analogous to the frictional and viscous losses in the column together with the column radiation resistance at the open end, the mass of the air column and the compliance of the air column, respectively.
When the speaker is located at one end of the column, the two circuits are connected in series to provide a further equivalent circuit.
The resonant frequency of the speaker is determined by a predetermined relationship set forth in the center of columns 5 and 6 of the patent and the combined equivalent electrical circuits have two principle resonant frequencies, one above and one below, and both less violent than the fundamental resonant frequency of the speaker. Of these principle resonances, the lower resonance extends the speaker response downwardly and the higher response lifts the trough of the falling low frequency response curve adjacent to the normal low frequency resonance. Between the two, where the speaker resonance would ordinarily have been, there is a "dip" which will be deep or shallow according to a number of factors, principally the various acoustic resistances present. This reference is fully incorporated herein and may be referred to for a more complete discussion. However, the speaker system is based on the provision of a column vertical axis at which the speaker is mounted above the open base of the column equal to a distance of 1/4 wavelength of the bass resonant frequency of the speaker.
In an article entitled "Build EI's Sub-Mini Speaker, Electronics Illustrated, November 1965, F. David Herman provides a step-by-step method of making a small speaker. In that article, a 4" speaker having a power rating of 3 watts nominal and 15 watts music power is employed in a stiff cabinet along with a contour network to match the speaker and the cabinet. The resulting speaker system provides a flat response, within 2 dB, from about 120 Hz to 14 kHz, with no prominent mid-range or upper end.
Herman states that one will not obtain knee-bending bass in that the same is virtually impossible with a 4" speaker in any enclosure.
The above may appear to be inconsistent with the teachings of the Australian patent; however, when one takes into account that the Australian patent refers to a larger speaker and a column radius of 71/2 inches, the differences in structure become quite apparent in that the larger speaker will normally produce a greater bass response, but with a reduction of higher frequency response due to larger cone weight.